The present invention relates generally to the field of cellulosic pulp processing, and more specifically to the processing of cellulosic pulp to prepare nanocellulose fibers, also known in the literature as microfibrillated fibers, microfibrils and nanofibrils. Despite this variability in the literature, the present invention is applicable to microfibrillated fibers, microfibrils and nanofibrils, independent of the actual physical dimensions.
Conventionally, chemical pulps produced using kraft, soda or sulfite cooking processes have been bleached with chlorine-containing bleaching agents. Although chlorine is a very effective bleaching agent, the effluents from chlorine bleaching processes contain large amounts of chlorides produced as the by-product of these processes. These chlorides readily corrode processing equipment, thus requiring the use of costly materials in the construction of bleaching plants. In addition, there are concerns about the potential environmental effects of chlorinated organics in effluents.
To avoid these disadvantages, the paper industry has attempted to reduce or eliminate the use of chlorine-containing bleaching agents for the bleaching of wood pulp. In this connection, efforts have been made to develop a bleaching process in which chlorine-containing agents are replaced, for example, by oxygen-based compounds, such as ozone, peroxide and oxygen, for the purpose of delignifying, i.e. bleaching, the pulp. The use of oxygen does permit a substantial reduction in the amount of elemental chlorine used. However, the use of oxygen is often not a completely satisfactory solution to the problems encountered with elemental chlorine. Oxygen and ozone have poor selectivity, however; not only do they delignify the pulp, they also degrade and weaken the cellulosic fibers. Also, oxygen-based delignification usually leaves some remaining lignin in the pulp which must be removed by chlorine bleaching to obtain a fully-bleached pulp, so concerns associated with the use of chlorine containing agents still persist. US Patent Publications 2007/0131364 and 2010/0224336 to Hutto et al; U.S. Pat. No. 5,034,096 to Hammer, et al; U.S. Pat. No. 6,258,207 to Pan; EP 554,965 A1 to Andersson, et al; U.S. Pat. No. 6,136,041 to Jaschnski et al; U.S. Pat. No. 4,238,282 to Hyde; and others exemplify these oxygen-based approaches.
Problems with these approaches include the need for a chelant and/or highly acidic conditions that sequesters the metal ions that can “poison” the peroxides, reducing their effectiveness. Acidic conditions can also lead to corrosion of machinery in bleaching plants.
The bleaching of pulps however is distinct from and, by itself, does not result in release of nanocellulose fibers. A further mechanical refining or homogenization is typically required, a process that utilizes a great deal of energy, to mechanically and physically break the cellulose into smaller fragments. Frequently multiple stages of homogenization or refining, or both, are required to achieve a nano-sized cellulose fibril. For example, U.S. Pat. No. 7,381,294 to Suzuki et al. describes multiple-step refining processes requiring 10 or more, and as many as 30-90 refining passes.
Another known method to liberate nanofibrils from cellulose fiber is to oxidize the pulp using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (“TEMPO”) and derivatives of this compound. US patent publication 2010/0282422 to Miyawaki et al., and Saito and Isogai, TEMPO—Mediated Oxidation of Native Cellulose: The Effect of Oxidation Conditions on Chemical and Crystal Structures of the Water-Insoluble Fractions, Biomacromolecules, 2004: 5, 1983-1989, describe this method. However, this ingredient is very expensive to manufacture and use for this purpose. In addition, use of this compound tends to chemically modify the surface of the fiber such that the surface charge is much more negative than native cellulose surfaces. This poses two additional problems: (1) the chemical modifications to cellulose may hinder approval with regulatory agencies such as the FDA in products so-regulated; and (2) the highly negative charge affects handling and interactions with other materials commonly used in papermaking and other manufacturing processes and may need to be neutralized with cations, adding unnecessary processing and expense.
As noted, ozone has been utilized as an oxidative bleaching agent, but it too has been associated with problems, specifically (1) toxicity and (2) poor selectivity for lignin rather than cellulose. These and other problems are discussed in Gullichsen (ed). Book 6A “Chemical Pulping” in Papermaking Science and Technology, Fapet Oy, 1999, pages A194 et seq., incorporated by reference. Additionally, the use of ozone or chemical agents as a bleaching pretreatment followed by a mechanical refining approach to liberate nanofibrils, entails a very high energy cost that is not sustainable on a commercial level.
Thus, it is an object and feature of the invention to provide an oxidative treatment process using ozone that is commercially scalable and requires use of significantly less energy than known methods to liberate nanofibrils from cellulosic fibers. Another advantage flowing from the invention is reduced corrosiveness and better environmental impact due to the avoidance of chlorine compounds.